This invention relates to a method for manufacturing metal material with an intermediate hollow and two solid end portions, particularly to one possible to simplify manufacture processes, to lower the cost of production and to strengthen the structure of products.
A conventional method for manufacturing a bike crank includes a step for preparing a sand-core embedded in an intermediate hollow in a mold of a crank. Then melted material is filled in the mold, and when the roughly shaped crank is to be taken out, an opening in the end section of the crank is bored for removing the sand core out of the mold. Then, the outer surface of the crank is ground smooth to finish the crank with the intermediate hollow. Such a method for manufacturing the crank is not only complicated in processes, but also increases the cost of production. Besides, boring an opening or a threaded hole for removing the sand core in the crank will weaken its strength.
Another conventional method for manufacturing a crank with an intermediate hollow is to preset a slot in one side surface of a crank body, which is to be closed up with a cover means welded together with the rim of the slot. Such structure of a crank is not integrated as one unit, weakened in its strength, and due to the varied materials of the crank and the cover means as well as possibility of expansion and contraction caused by weather, so the joint section may be liable to split.
The objective of the invention is to offer a method for manufacturing metal material with an intermediate hollow and two solid end portions. A first step of the method is to prepare a roughly-shaped metal material having a first inner solid end portion, and a bar body having a lengthwise intermediate hollow, and an expanded conical portion set around an open end hole of the bar body. Next, the expanded conical portion is compressed and shrunk to move towards the open end hole of the bar body and closes it up, forming a second solid end portion; then, the intermediate hollow is closed up by the second solid end portion, and a finished crank of metal material with an intermediate hollow and two solid end portions is obtained.
Another objective of the invention is to offer a method for manufacturing metal material with an intermediate hollow and two solid end portions, which is simple in manufacture, quick in shaping, integral as one unit and sure to strengthen the structure of metal material.